1. Field of the Invention
This invention relates to improvements in the method of manufacturing cement clinker from cement raw slurry. The invention also relates to an improved plant for practicing the method of the present invention.
2. Description of the Prior Art
In the manufacture of cement clinker, which is an intermediate product in the manufacture of cement in a rotary kiln, a distinction is made between the wet method and the dry method carried through in a wet kiln and a dry kiln, respectively. In the former case the cement raw materials are fed to the rotary kiln in the form of an aqueous suspension, or a so-called raw slurry. In the latter case the cement raw materials are fed to the rotary kiln in their dry condition, usually in the form of a powder described as raw meal.
The method that is more advantageous in any given case generally depends upon circumstances. However, the tendency is towards preferring the dry method to the wet method, primarily due to the fact that the dry kiln has a heat economy superior to that of the wet kiln because a substantial amount of water has to be evaporated in the latter. Such evaporation of water requires a substantial quantity of fuel.
In certain cases the cement manufacturer may for some reason find it desirable to use a dry kiln for the manufacture of clinker, while using cement raw slurry as a starting material. This is best described as a combined method. In such a case the raw materials which may be used are perhaps in their natural state, so moist that it is more rational to increase the water content to such an extent that they may be best transported to the works as a slurry by being pumped through pipings rather than to have their water content reduced. In other cases the cement works may be located at such a large distance from the quarry where one or more of the raw materials are obtained, that the least costly way of transportation to the cement works will be to add water to the dry raw material and to pump the resulting slurry of raw material thus produced through pipings.
The aforesaid combined method of manufacturing cement clinker may advantageously be carried through by subjecting the raw slurry to a suspension drying, preferably a spray drying, since it will thereby be possible to reduce the temperature of the exit gases after drying to a larger extent than would be possible if the drying were preformed in the rotary kiln.
To operate as intended in combination with a conventional dry kiln working with the usual surplus of air, the spray dryer requires a supply of gas of a rather high temperature. The temperature required is generally in the range of approximately 900.degree.-1200.degree.C. On the other hand the spray dryer is also capable of withstanding these rather high temperatures which are actually within the normal temperature range of the exit gases from such a dry kiln. The spray dryer makes it possible in a single stage by means of hot gases to convert cement raw slurry having even a rather high water content, for instance 30-40% by weight, to practically water-free raw meal which is ready to be introduced into a rotary kiln.
The aforesaid combined method seems thus to offer substantial advantages which, however, are reduced to some extent by the fact that the raw meal is not preheated prior to being introduced into the rotary kiln. The raw meal produced by the spray drying has a temperature of about 100.degree.C. and it would have been desirable if it were substantially higher. However, this drawback may be remedied by effecting a heat exchange between the rotary kiln gases and the raw meal in a suspension preheater before the said gases are used for spray drying of the raw slurry. This solution is in certain circumstances unsatisfactory; although the raw meal would, in fact, be preheated to, say, 600.degree.-800.degree.C., the corresponding temperature required of the rotary kiln gases would at the same time have to rise substantially, say, to about 1300.degree.C. This would be unacceptable in view of the materials ordinarily used for the structures at the transition between the rotary kiln and the stationary smoke chamber.
I have invented a method and a plant for manufacturing cement clinker from raw materials in the form of cement raw slurry which overcome these disadvantages. An essential and surprising feature of the invention consists in carrying through the initial heat treatment of the raw slurry such that the hot exit gases from the rotary kiln are caused at first to give off heat for preheating at least part of the raw meal produced by the spray drying. Secondly, they contribute to carrying through the spray drying of the raw slurry; the remaining amount of heat necessary for the spray drying being procured from other sources, preferably from hot gases generated in a separate furnace. By thus supplying extra heat for the spray drying from a supplementary heat source, it is possible to achieve the improved desirable working conditions to which my invention is directed.